Currently, tipping vehicles such as utilities, trucks, trailers, pick-ups, dump bins, style sides and the like are provided with a .hydraulic or pneumatic cylinder which is located generally between the chassis and the tipping platform or tray- Operation of the cylinder causes the tipping tray to pivot between an inclined tipping position and a horizontal rest position. A disadvantage with such cylinders is the relative high cost of installation and the need to provide a separate hydraulic or pneumatic system including pumps, high pressure hoses, tubes and the like.
More recently, inflatable bags have been used to substitute the above known cylinders. The bag is inflatable (usually by air or exhaust gasses) and has a lower end secured relative to the chassis and an upper end which is secured relative to the tipping tray. It is also known to inflate these bags using the exhaust system of the vehicle through a diverter valve in the exhaust system which diverts exhaust air through a suitable hose line into the gas bag.
Such earlier gas bags are disclosed in our earlier Australian patents 579830 and 619811 filed in 1984 and a second type is illustrated in U.S. Pat. No. 5,067,774 to Trowland.
Earlier proposals for gas bags suffer from many disadvantages which to date has made them commercially unattractive, and in practice unsuitable for installation in pick-ups and style sides light trucks.
For instance, for use in commercial utilities, trucks and trailers, it is found that the area available for air bag installation is relatively small. This is because the chassis includes many obstructions which are difficult or uneconomic to re-position including petrol tanks, chassis rails and the like. Furthermore, design rules for vehicle impact constrain the layout of the chassis components. It is found that in practice, the only readily available position is relatively close to the rear hinge point where the tipping tray is hingedly connected to the vehicle chassis.
However, a difficulty in placing an inflatable gas bag close to the pivot point is the larger pressure required to inflate the gas bag and particularly to cause the tip tray to pivot when under load conditions. That is, a general rule of thumb is that the closer the gas bag is to the pivot point, the more pressure is required, the stronger the gas bag material has to be and the more expensive it is to manufacture. This has been a consideration which has deterred many known gas bags from commercialization. However, as discussed above, this is the area of least obstruction and therefore the area where a gas bag can be most easily fitted to a range of vehicles.
Other considerations which hitherto have caused difficulties to existing gas or air bags is the relatively small amount of clearance space available between the tipping tray and the chassis due to the vehicle design. This meant that the gas bag was required to be of relatively small size such that in the collapsed state it could be fitted in this relatively small space, for instance, between the chassis rails.
However, gas bags fitted further away from the hinge point of the tipping tray, are required to be of larger size in order to allow the tray to be tipped to a useful angle which should be 45.degree. or more. Furthermore, at such a tipping angle, the bag should be stable, especially under load conditions, and there should be no tendency to buckle or deform.
Another consideration is the tipping rate, that is the rate at which the tipping tray can be raised or lowered. This is determined by the volume of gases flowing into the bag and the volume of the bag itself. It can be seen that for an acceptable tipping rate, the bag volume should be kept as small as possible and/or the volume of gas flowing into the bag should be increased. The volume of gas can be increased by revving the vehicle engine (if exhaust gases are used), however, this is undesirable in terms of noise pollution. Most operators would like the gas bags to be inflated while the vehicle is idling or at an engine speed slightly above idle. Therefore, the other consideration is to minimize bag volume which in turn teaches away from the use of the bag at a distance from the hinge point which requires larger bag volumes and sizes.